The microscopic structure of a powdered white pine is as follows: The epidermis (1) consists of reddish-brown masses, irregular in outline. The outer parenchyma cells are of a bright-green color, owing to the presence of chlorophyll. (The above elements are not usually found in the rossed bark.) The parenchyma (3) with starch usually occurs in longitudinal sections accompanied with sieve cells. Often the tissue separates transversely, showing the medullary rays (4) with their granular cell contents (9) and the inner parenchyma cells filled with starch and the surrounding sieve cells. The crystals are nearly perfect cubes and occur singly (5) or in groups (6). On the longitudinal section of the bark the crystals occur in parenchyma cells surrounded by a reddish cell content and form parallel rows which are very characteristic. The resin occurs either as white, angled fragments (7) in a water mount, or as globular mass (8) or as reddish-brown pieces (10). The starch is very abundant and is distributed through the field. The diagnostic grain is lens-shaped, with a cleft hilum, which is nearly straight, or slightly curved, and runs parallel to the long diameter of the grain. The addition of ferric chlorid T. S. will show the presence of tannin by forming a dark coloration. The identification of the starch is facilitated by the addition of a weak Lugol’s solution, which imparts a blue coloration to the starch grain. The form, amount, and distribution of the cells composing the bark differ greatly in different plants. In cramp bark the cork and phellogen cells are very large, while in cascara sagrada the phellogen and the cork cells are very small. [Illustration: PLATE 104 POWDERED WHITE PINE BARK (_Pinus strobus_, L.) 1. Epidermis. 2. Parenchyma cells. 3. Parenchyma with starch. 4. Medullary rays. 5. Solitary crystals. 6. Solitary crystals and tannin. 7, 8 and 10. Resin masses. 9. Starch.] In canella alba bark the periderm is composed of stone cell cork or stone cells arranged in superimposed rows, which form the outer layers of the bark. In white oak and most barks from woody trees the periderm consists of lifeless parenchyma, medullary rays, sieve cells, bast fibres, and in some cases stone cells and of phellogen cells. In young wild cherry, cascara sagrada, and frangula are several layers of tangentially elongated collenchyma cells with chlorophyll. In the older barks of the above and in many other barks no collenchyma cells occur. In cramp bark and in tulip tree bark the outer layers of the cortical parenchyma cells are beaded. In most barks there is no beaded walled parenchyma. The outer layers of most cortical parenchyma cells are tangentially elongated while the inner parenchyma cells are mostly circular in outline.